The present invention relates to a new and improved punch and counter punch plate which are fabricated from chromium containing alloys and employed, for example, in punching or stamping machines.
Punch and counter punch plates are tools which are machined on all sides by shaping, milling or grinding and which are provided with bores and recesses aligned with the punching or stamping machine for affixing these tools at such machine and for determining or fixing cutting lines.
These tools or punch and counter punch plates must have high hardness and must meet the special requirements with regard to plane parallelism and flatness or planeness.
Raw materials or blanks employed for punch and counter punch plates are preferably hardened plates made from low-alloy or medium-alloy heat-treatable steels or tool steels such as, for example, DIN (German Industrial Standard) 50 CrMo 4, VEW (Vereinigte Edelstahlwerke) Material Number 1.7228 and AISI (American Iron and Steel Institute) 4150 having a case hardness in the range of 45 to 53 HRC. According to experience, these steel alloys are highly resistant to abrasion or wear when hardened or tempered.
These steel alloys, however, are not resistant to rust. When a film of moisture forms on the surface, for example, by climatically caused condensation of air humidity during changes of temperature, a thin, brownly colored punctiform coating of rust forms on the surface in the beginning stage. This coating of rust would contaminate products made of paper, cardboard or corrugated board and therefore must be removed by time-consuming maintenance before beginning production. The removal of the coating of rust is accomplished by wiping or polishing depending on the thickness of such rust coating. In addition to the expense for cleaning maintenance, an additional reduction in the service life can occur due to unfavorable environmental conditions such as, for example, in tropical climates, at locations near the sea or by storing the tools in the open.
A special type of corrosion, namely stress corrosion cracking, further appears in these punch and counter punch plates because, as can be easily imagined, especially high stresses occur in the punch and counter punch plates due to the punching pressures.